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Abstract Plagioclase ultraphyric basalts (PUBs) are a class of mid‐ocean ridge (MOR) lavas found in a variety of ocean floor environments, are characterized by abundant (15–40 volume %) plagioclase megacrysts and a diverse trace element and isotopic signature. Paradoxically, we never see lavas erupted on the seafloor that are in equilibrium with these PUB megacrysts. Based on petrographic evidence, melt inclusion composition, and new data on depth of entrapment calculated from CO2 contents in plagioclase‐hosted inclusions, many of the megacrysts formed at upper mantle pressures (∼3–7 kbars). To constrain the composition of the parent magmas of the plagioclase megacrysts, we conducted a series of experiments at 5 and 10 kbars using mid‐ocean ridge basalts glasses as starting materials. The experimental results were consistent with the presence of a pseudoazeotrope in the anorthitic segment of the plagioclase + basalt pseudobinary. This has the effect of dropping the anorthitic end of the feldspar loop, lowering the solidus for upper mantle conditions, and driving evolving magmas toward higher Ca. As magmas rise and pressure drops, the pseudoazeotrope disappears, and the feldspar loop at the high‐An end rises, causing those magmas to undergo decompression crystallization of plagioclase and resorption of olivine. Therefore, the conditions which generated the magmas from which the megacrysts form disappear as the magmas rise and magmas evolve toward lower Ca, Mg (as we normally assume during plagioclase + olivine crystallization). In effect, the phase equilibria conditions that allow for the generation of such liquids also prevent them from being erupted as lavas.